Environmental aspects of pulp washing

Brownstock washing is performed to separate dissolved organics and spent cooking chemicals from pulp. As such, any method of characterizing the concentration of organics in the liquid associated with the pulp mat may be used to represent the performance of a brownstock washing system. Corrections must be made to properly determine the effect of mat consistency on these relative types of tests. Brownstock washing may significantly effect the amount of bleaching chemicals required to obtain a specific pulp brightness. As the amount of applied bleaching chemical is increased by increased carryover of dissolved organics from brownstock washing, a commensurate increase in the formation of chlorinated organics occurs. Methods of improving environmental performance of a brownstock washing system typically focus on optimization of washers performance. Parameters of interest in the evaluation and improvement of brownstock washers include the judicious application of shower water, appropriately sized and arranged drop legs, and the proper number and location of shower bars. Another important environmental concern typically found within a brownstock washing operation is the screen room. Closing an open screening system may substantially reduce the amount of BOD and color sent to an effluent treatment system.